Worldwide, as the crude oil for processing is becoming heavier and poorer in quality, it has become a common view that, for the processed distillate oils, removal of aromatic compounds in diesel fuels may help to improve the cetane value of diesel, removal of aromatic compounds in aviation kerosene raw materials may contribute to increase the smoke point of products, and removal of aromatics in lubricant base oil raw materials may improve the antioxidant stability of products.
As compared with non-noble metal (cobalt, molybdenum, nickel, and tungsten) sulfide catalysts, noble metals prominently demonstrate more superior hydrogenation performance, and therefore noble metal catalysts are widely used in dearomatization of distillate raw materials. Noble metal catalysts exhibit good dearomatization performance at low temperature. However, the hydrogenation performance of distillate processing by using noble metal catalysts is usually related with their sensitivity to sulfur-containing compounds, while noble metals normally have poor sulfur resistance and therefore noble metal catalyst poisoning tends to occur during the distillate processing. Moreover, distillates from some refineries have high content of sulfur-containing compounds, and it is unqualified to directly use noble metal catalysts for dearomatization without desulfurization treatment of the distillates in advance.
U.S. Pat. No. 3,943,053 discloses a selective aromatics hydrogenation catalyst, and states that the catalyst prepared by the method thereof can maintain its hydrogenation performance for a very long time when used for processing sulfur-containing raw materials, wherein the catalyst employs γ-Al2O3 having high specific surface area as a carrier loaded with platinum and palladium. A crucial issue in the preparation of such a catalyst is to impregnate the carrier with platinum and palladium simultaneously in the same aqueous solution, but the distribution state of the noble metals in the catalyst is not described.
U.S. Pat. No. 5,308,814 discloses a number of carrier materials in a noble metal dearomatization catalyst composition, and describes that a carrier is prepared from Y zeolite and heat resistant inorganic oxides (for example, silica, alumina, or silica and alumina) and loaded with platinum and palladium, wherein the weight ratio of Y zeolite in the carrier is 10 to 90%. Thus, it can be deduced that such a material has strong crackability and is not suitable in the field of hydrofining.
U.S. Pat. No. 4,849,093 discloses a two-step dearomatization saturation method using a hydrogenation catalyst comprising group VIII metals or group VIB metals or combination thereof in a non-noble metal catalyst, but the distribution state of active components in the catalyst is not described in this patent either.
U.S. Pat. No. 3,869,522 discloses an aromatics hydrogenation method in which a carrier containing 30 to 90 wt % silica, 10 to 70 wt % zirconia, and 0 to 25 wt % alumina is prepared. In this patent, platinum is preferably used as the active metal, or its combination with other noble metals may be used as the active metal. Raw materials with a sulfur content exceeding standard may be catalytically desulfurized. It is acknowledged in this patent that the exposure of noble metal hydrogenation catalysts to sulfur-containing compounds leads to sulfur sensitivity to such sulfur-containing compounds, but it is not yet recognized that the noble metal hydrogenation catalysts can be protected by altering the distribution state of noble metals in the catalyst.
Currently, it is conventional in the industry to prepare an impregnation solution having a fixed concentration of active metals and impregnate a catalyst carrier by saturated or oversaturated impregnation followed by drying and calcination to prepare a catalyst. U.S. Pat. No. 4,399,058 introduces a method for preparing a hydrogenation catalyst in which group VIB and group VIII metal salts are mixed with aqueous ammonia followed by further addition of aqueous ammonia to adjust pH to a certain value so as to prepare a noble metal solution at a fixed concentration, and an inorganic oxide carrier is saturated impregnated with the metal solution, dried and calcinated to provide a hydrogenation catalyst.
European patent EP 0 204 314 describes a method for preparing a hydrogenation catalyst with an uneven distribution of noble metal components. In the preparation process, a stepwise multiple-impregnation approach is employed to load the noble metal components, i.e., a carrier is impregnated with a solution of active component A, and then washed, dried, and calcinated; subsequently, it is impregnated with an impregnation solution of active component B, and washed, dried, and calcinated again. By using a preparation method including stepwise washing, drying and calcinating, the noble metal content within the catalyst particle is made higher than the metal content on its surface, and the life span of such catalyst with an uneven distribution is extended as compared to a catalyst with an even distribution. However, due to the complexity of this preparation method and loss of noble metals in the preparation process, the cost for preparation of such a catalyst is rather high.